In a small saucepan cook mushrooms and onion in margarine or butter over medium-low heat for 10 to 15 minutes or until onion is very tender, stirring occasionally. Set aside.
Meanwhile, in a mixing bowl stir together the sour cream, bread crumbs, mustard, salt, and pepper. Stir in ground beef, pork, turkey, or chicken. Shape meat mixture into two 3/4-inch-thick patties.
To broil, place meat patties on the unheated rack of a broiler pan. Broil 3 to 4 inches from the heat for 8 minutes. Turn and broil 8 to 10 minutes more or until an instant-read thermometer inserted in side of patty registers 160 degrees F. Place a slice of cheese atop each burger. Broil about 1 minute more or until cheese begins to melt.
Or, to grill, place meat patties on the rack of an uncovered grill. Grill directly over medium-hot coals for 8 minutes. Turn and grill for 8 to 10 minutes more or until no pink remains. Place a slice of cheese atop each burger. Grill about 1 minute more or until cheese begins to melt.
To serve, place each burger on the bottom half of a toasted bun. Top with mushrooms and onion, then a bun top. If desired, serve with mustard. Makes 2 servings.
Go picnic-style with baked beans and glasses of lemonade.